Tone control circuit



lApril 25, 195o M. H. MESNER 2,505,254

\ TONE CONTROL CIRCUIT Filed Sept. 7. 1944 Tlc'l.

ATTORN EY Patented pr. v257, 1.95,()v

TONEUONTROL CIRCUIT Maxl-I.' Mesner, Princeton, Nk .I assignor to-Radio Corporation of America, a, corporation oiDela,`

Ware

ApplicationSepteinher 7, 19514, Serial No..553,024

11 Claims. 1

present` inventionfrelates .generally toirequenoy :discriminatory f .devices, and: more par: ticularly toA an .improved audio. frequency tone control.

In, audio -frequency signal transmission sys.-

such; .as-.those.used in 1 radio. receivers, itL'is usual.l practice tov gprovide, :a tonev control device for attenuating-v-the.high audio. frequenciesvto, .a greater or ,lesservdegree Fory .speeclnand other modulationsignals, where-.too :much pass.` response interferes with -ntelligfilmllity, Aii-may .be

desirable also to. limiti-the response .-.over` the .low audio f frequency, frange. -1 Means. forl Llimiting ethe bassy response haye, also,- heretofore been. pro.-

posed.

.One-of thefmein .ohjectsof my presentimention is. to; combine in yone.improved .deyicef which is continuously.; variabler theliigh audio ,fre-

euenQ-y and 1 W-audioy frequency attenuation controls; y

Another ;obiect-of,-my invention .is to, provide a ihifgtrl-y simplied form of continuouslyyariable tone control :devicefwhchconsistsfessentiallyof a three terminalgpotenticmeter; two terminals .of the potentiometer being used irl-cooperation with the movable potentometerslider vas a thigh audio mentioned (terminals .andthe third terminalmay belused to attenuate the,lowfaudiofreguencycmrents.`

ncontrolgdevice.which may be economicallyrmanuactured :and fassembled in anv audio, `frequency signal transmission network.

Other featureszfoff .,myyrinvention .Willtestv be understood` byrefercnceto the .following description. taken Hin connection Awithy thedrawingin which I haveindicated 1diagra;mmat-ically`r alcircuit organization;whereby my inventiQnniay-he i carried intov eiect.

l2in the drawing-z nig. 1- -diagrammaticaIl-y shows-.arr :audio-.fre-

`quency signal.v` .transmissionf-fx1etworkf' embodying myinvention;

Fig; J2 yshowsfvariousxresponseqzcurves secured for various settings -pfffthentone controllv device;

.andv t pig.;3.illustratessaidialtwhichimay be :employed 2 in .coniunction with the Vadjustable element of the tone control device.- o

The present invention' is not limited 'in any way :teosignali transmission-:in the audio fre-i quency range )cutis-generally applicable to signals in-av wide range ci frequencies. Furtherincre,.tliefsource ofv signals. for thenetwork may he :the .,detector=.output=.oi .a; radio receiver, the microphone of @a publicaddress system, the pickup of a record reproducing device, an audio frequency signaldistribution-line or-eVen-theivideo signal transmission network `of atelevisiorrd re#- oeiveri For-thepurposeof -thepresent application; lnovveverl,V it is assumed-that the sourceof signalA voltage isV an'aud-iofr`equencyr voltage source Tne'numerai -i' designatesfan electron discharge tube AWhich -iswshownY -bytway `ofV illustration as 'ployed The control `gri-d 2 of tubeal is'connected through ya direct; currentfblocking condenser 3 to the lhigh 1 alterna-tingL -potentialinput lead Yll. The numeral 5 denotes the low potential or ground lead ofi-the inputcircuit: The cathode 6 *of tube i is connected toground throughlthe usual gridbias network 1: The grid return-resistor -S is connected from the output terminalof condenser Sito grou-ind,y The plate -9 of tube YI Vis connected to thev +B terminal of Vafsourceoi di"- Yreet-current (notshown) 'through-thefoutput'load resistor it, Those skilled inthe art of radio communicationare fully awarefof xthe nature of Jthe-direct current supply source iwhich may-be employed with asystem ofthe-*type shownf-in Fig. 1.

The audio Afrequency signal Y'voltage applied `betweenthe input'grid 2fand cathodei is Iam'- pliledby tube' i, and the ampliedaudioffrequen'ey signal voltage developedacross" load rre'- sistor 23` isappliedhetween the `input electrodes of ,thefollowing electron dischargetube H.v The tuloe4 ii, 'whichymaygbe of. any vdesiredltype,A has its inputveiectrode `or grid' i12 connected;V to `the vdirect current blocking condenser I3; The input terminal of thelatter isconnected tolthe plate `endro resistor it., The cathode ld' oftube I Vis "instable slider. .mand the lower .end of lresistor R. The. grid, return resistor. path periormsa .douhie .function in Emy.. invention.. and it ,is an essential-'part of: .the tone controldevice.. 'Ehe upperendet resistor R is connected hymen.-

.densersi 'i to the input. terminal Kof condenser.v I3. 60 ff It; wil1zbe.=seen, therefore-that ladjustin-ent.-.of

slider or tap I@ along resistor R varies concurrently the magnitude of resistance in series with condenser Il between the plate circuit of tube I and ground, and also the magnitude of resistance between the grid circuit of tube II and ground.

The plate circuit of tube I I is completed in thev The signal voltage amplified by.

usual manner.

tube I I is developed across load resistor I The amplified signal voltage transmitted to output leads i9 will have its frequency response charac- 2 teristic dependent on the adjustment of slider I6. The utilization means may be a loudspeaker, amplifier or the like. the following values of the above-recited elements Ahave been used in a successful embodiment of the invention. My invention, however, is not limited .to use of these particular Values.

I=33,000 ohms R1=100,000 ohms Y R=1 megohm Plate resistance of tubeV l :8,000 ohms Condenser I1=0.025 microfarad Condenser 53:0.005 microfarad Resistor Explaining, now, the functions of the elements of the tone control device, it is pointed out that the latter consists of a simple potentiometer having three terminals. Two of these terminals a and b are connected to provide a variable attenuation path for the higher audio frequency cur- .analyzed in the light of the experimental curves of Fig. 2 and the illustrative control dial of Fig. 3. 4In Fig. 2 I have shown various frequency response curves experimentally secured for various settings of the tone control adjustable element I. Response in decibels is plotted against Frequency cycles per second.

3 In Fig. 3 there is shown an illustrative dial that may be used in conjunction with the adjustable element I6. Let it be assumed that potentiometer slider I6 is rotated in an arc in sliding contact with the resistor R. In such case the indicator .or pointer 2% would be mechanically coupled to element i6 for motion therewith. At position 1 of slider I6 (extreme counterclockwise position) the slider will be located at the upper end of resistor R. At position l in Fig. 3 (extreme clock- Wise position) the slider is located at the terminal c of resistor R. Position 5 of the indicator 20 corresponds approximately to the midway position of slider I6 on resistor R. It will be noted .that position 1 is the Treble attenuation point;

position 5 is the High delity setting; setting 10 is the Bass attenuation adjustment. Fig. 2 relates these three settings on the frequency response curves. Fig. 2 are numbered to correspond to the tone vcontrol settings of Fig. 3.

It will be observed that when the slider I6 in Fig. 1 is adjusted to the upper end of resistor R, there will occur maximum bypassing of higher audio frequency currents. In this position of the slider the condenser Il connects the plate circuit of tube I directly to ground, and no portion of resistor R is included in circuit with the condenser. When the lslider is adjusted in the Further, the response curves of By way of specific illustration, v

eounterclockwise path of the potentiometer range, higher audio frequencies are attenuated to a greater or lesser degree depending on the amount of resistance to ground that is placed in series with condenser I'I. For the higher audio frequencies the impedance of condenser Il' plus the resistance in series therewith becomes low enough sufficiently to lower the plate load resistance I0. The impedance of condenser Il at higher than any chosen cut-off frequency should be nearly as small as, or smaller than, the impedance of the load resistor Il). That is, of course, true for counterclockwise positions of slider i6. This reduces the gain of tube I for the higher audio frequencies.

The value of resistor R is chosen so that when the slider I6 reaches the midpoint on R, the resistance in series with condenser Il is sumciently high in comparison to the load resistance i0 that its shunting effect even at higher audio frequencies becomes negligible. This midpoint on position 5 of indicator 20 is the high fidelity region, where the tone control has no effect at either the high or low frequency end. This is best illustrated in Fig. 2 wherein the frequency response curve corresponding to position 5 of indicator 20 possesses substantially uniform transmission of audio frequency currents above lcycles with minimum attenuation of currents below 100 cycles.

If the slider I6 is moved closer to the lower end c of the resistor R, that is clockwise, the resistance from grid I2 to ground becomes less. For the midpoint-position of slider IS, the resistance of R1 plus half of the resistance of R is the factor which controls the value of condenser I3. The capacity of condenser I3 is chosen so that the time constant (i. e. the RC product) is great enough to pass low audio frequencies down to the lowest audio frequency required for high fidelity. As the maximum clockwise position is reached the time constant becomes less thereby causing the low frequency range to be curtailed. At the extreme clockwise position of slider i6, that is position 10 of indicator 20 in Fig. 3, the bass character is determined by the magnitude of coupling condenser I3 and resistor R1. The magnitude of resistor R1 is thus chosen to give the maximum frequency range reduction required. It is preferred to choose the magnitude of R1 so that it is comparatively large with relation to the load resistance I0. The reason for this relation is that the tone control device will have a minimum eifect on the general volume level. This can best be accomplished by choosing the resistance of load I0 as low as the amplification requirements will allow. The impedance of condenser I3 at low audio frequencies should be as great, or greater than, that of R1. Hence, at such frequencies there will be a definite amount of voltage division at the grid terminal.

From the experimental curves of Fig. 2, it will be seen that when slider I6 is adjusted to position 1 in Fig. 3 there will be maximum attenuation of the higher audio frequencies, with no effect on the lower audio frequencies. Adjustment to position 2 of Fig. 3 results in substantially the same frequency response curve. Successive adjustments to positions 3 and 4 result in successive decrease of attenuation in higher audio frequencies. At the midpoint 5, as stated before, there occurs optimum transmission over the entire signal frequency range. Further adjustment of the slider to positions 8, 9 and l0 in succession results in increased attenuation of 'the lower audio vtfequenciels, but with substanffti'aily no "effect -on tleliighe'r audio frequencies. `It will, therefore, Ibe l's'ee'n that the tone control -devicef'of nypr'esent inventionzprovides `a median fs'ttingfor high ffid'elity signal transmission 'with :increasing Euattenuation 'of higher audio frerdencies upon movement of the tone control i'n one direction from the median setting, and with ineiisiig attenuat-ihn of lower audio frequencies tupon movement of l*the tone control device in the oppsite Ydire'c'tiz'jn' -from the median setting.

While I have indicated and. described a system for carrying my invention into eiiect, it will be apparent to 'one Askilled in the art that my one output terminal and the other input and output terminals being connected to a common point; "a frequency controlnetwork which infu'des a compensating capacitor connected between said one input terminal and one end of a potentiometer having an adjustable contact arm, and a fixed resistor connected between said one output terminal and the other end of said potentiometer, said contact arm being connected to said common point.

2. In .combination with an audio frequency signal amplifier tube provided with an output load, a second amplifier tube provided with at least a cathode, a control grid and an output electrode, a first condenser of low impedance to high audio frequency currents coupling the plate end of said output load to said control grid, a potentiometer having an adjustable contact connected to ground, a second condenser of relatively low impedance to higher audio frequencies connected from the plate end of said output load to one end of said potentiometer, and a resistor of fixed magnitude connected from said control grid to the other end of said potentiometer whereby said control grid is returned to ground through said fixed magnitude resistor and a portion of said potentiometer.

3. In combination with an audio frequency signal amplifier tube provided with an output load, a second amplifier tube provided with at least a cathode, a control grid and an output electrode, a first condenser of low impedance to high audio frequency currents coupling the plate end of said output load to said control grid, a potentiometer having an adjustable contact connected to ground, a second condenser of relatively low impedance to higher audio frequencies connected in series from the plate end of said output load to one end of said potentiometer and through said contact to ground, a resistor of fixed magnitude connected from said control grid to the other end of said potentiometer whereby said control grid is returned to ground thro-ugh said fixed magnitude resistor and a portion of said potentiometer, the overall resistance value of said potentiometer between the ends thereof being very high compared to that of the fixed resistor whereby the lower audio frequencies are not attenuated until after a substantial portion of the resistance of said potentiometer is 6 included in Aser-ies with :the second 'condenser to gound.

4. AIn vcombination with an audio frequency signal vamplifier tube provided with an 'output `load resistance, a'isecond amplifier tube provided with at 'least a cathode, a controllgrid andan output "electrode, a first condenser of low impeda'nce Yto high faudio frequency 'currents cou- Spling the 'plate -end 'of said output load .resisttance to 'said control grid, a potentiometer having an adustable contact connected to ground, a second condenser of relatively low impedance to higher audio frequencies connected in series `irinthe plate 'end of `said output load to one end `fof said potentiometer and through said contact to ground, a resistor of fixed magnitude connected from rsaid control ugrid Ito thefot'her end "of said 'potentiometer whereby said control y,grid is 'returned to ground through said fixed "magnituderesistor and a portion of said potentiometer, said fixed magnitude resistor being comparatively large relative to said output load resistance so that Vvariation of `said potentiome- 'ter will have a minimum effect on the volume level vat the voutput of the second amplifier tube.

5. In 'a system as ydefined in claim 2,`said\first condenser having an impedance at low=audio 'frequencies which is at `least as great as the im- .f'peda'nce of said resistor fof iix'ed magnitude.

6. In a system asde'ined Ein ic'l'a'im 2,saicir'second condenser having an impedance which, at higher than any chosen cut-off frequency, is at least as small as the impedance of said output load resistance.

7. In combination with an audio frequency signal transmission network provided with a pair of input terminals and a pair of output terminals and a .coupling condenser connected between one of said input and one of said output terminals, an adjustable attenuation network connected in shunt across the transmission network, said attenuation network consisting of a compensating condenser having a relatively low impedance at the upper end of the audio frequency range connected between said one input terminal and one end of an adjustable potentiometer resistor having a movable contact connected to the other input and output terminals, a second attenuation circuit including said coupling condenser, said coupling condenser having a relatively low impedance to said high frequency end of the signal frequency range, and a connection between said one output terminal and the other end of said potentiometer, the magnitude of the resistance included in said second attenuation circuit being directly dependent upon adjustment of said movable contact, and the impedance of said coupling condenser at low audio frequencies being chosen to be nearly as great as, or greater than that of the resistance included in said second attenuation circuit at any Yposition of said movable contact.

8. In combination with an audio frequency signal amplifier tube provided with an output resistor, a second amplifier tube provided with at least a cathode, a control grid and an output electrode, a first condenser of low impedance to high audio frequency currents coupling the plate end of said output resistor to said control grid, a second condenser of relatively low impedance to higher audio frequencies connected in series with a first variable portion of a potentiometer resistor from the plate end of said output resistor to ground, a resistor of fixed magnitude connected from said control grid to said poten tiometer resistor whereby said control'grid is returned to ground through said resistor of fixed 4magnitude and a second variable portion of said potentiometer resistor, the overall resistance of said .potentiometer resistor being chosen very high compared to that of the xed magnitude resistor whereby the lower audio frequencies are not attenuated until after a substantial portion -of the resistance of said potentiometer resistor is included in series with the second .condenser `to ground, said fixed magnitude resistor also being large compared to said output resistor.

9. The combination set forth in claim 1 in which said compensating capacitor has a relatively low impedance at the high frequency end of the signal frequency range and in which the overall resistance val-ue of said potentiometer is such that when said contact arm is centered on said potentiometer the resistance in series with said compensating capacitor is sufficiently high that the loading eiect of said compensating capacitor on said network is neligible.

10. The combination set forth in claim 9 in which the time constant of the resistance of said xed resistor plus one-half the resistance of said potentiometer with the capacitance of said coupling capacitor is so related to the lowest frequency of the signal frequency range that said lowest frequency is passed through said network substantially without attenuation.

Y11. The combinalton set forth in claim 1 in Vwhich the time constant of the resistance of said `fixed resistor plus one-half the resistance of said potentiometer with the capacitance of said coupling capacitor is so related to the lowest frequency of the signal frequency range that said lower frequency is passed through said network substantially without attenuation.

MAX H. MESNER.

REFERENCES CITED The following references are of record in the file of this Ipatent:

UNITED STATES PATENTS l Date 

